The present invention relates to an apparatus for coding an image signal.
In handling continuous-tone image information, if the image information is digitized as it is, the amount of information becomes very large, so that the amount of information is generally compressed by coding the image information.
Although there are various techniques of coding image information, as a typical technique of coding a continuous-tone image, a transform coding method is known, which is disclosed in, for instance, Hideo Hashimoto: An Introduction to Compression of Image Information "Image Coding Algorithms II--Transform Coding ", The Journal of the Institute of Television Engineers of Japan, Vol. 43, No. 10 (1989), pp. 1145-1152.
Such a transform coding method will be described with reference to FIG. 1 showing a basic block diagram thereof.
Reference numeral 101 denotes a blocking device for detecting a rectangular area in an image as a pixel block 102 from a digital image signal 100; 103, a converter for performing orthogonal transforms with respect to the pixel block 102 and outputting the same as transformed coefficients 104; 105, a coefficient selector for selecting a specific coefficient from the transformed coefficients 104 and outputting a selected transformed coefficient 106; 107, a quantizer or quantizing the selected transformed coefficients 106 and outputting a quantization coefficient 108; and 109, an encoder for encoding the quantization coefficient 108 and outputting encoded data 110 to a transmission line 111.
Next, an operation of the system shown in FIG. 1 will be described. In the system of FIG. 1, coding processing comprises transform processing, information reduction processing, and code assignment processing.
In the transform processing, in the case of an image signal, a two-dimensional orthogonal transform is carried out in which the correlation between horizontal and vertical directions is utilized. The pixel block 102 including M pixels and N pixels n the horizontal and vertical directions, respectively, is formed n the blocking device 101, and a one-dimensional orthogonal transform is carried out independently in the horizontal and vertical directions in the converter 103. In the converter 103, a linear transformation is performed according to the following expression (1): EQU Y=A.sub.N .times.A.sub.M.sup.T ( 1)
where X is the pixel block 102 having N rows and M columns; Y is the transformed coefficient 104; and A.sub.N and A.sub.M are orthogonal transform matrices n the N-th and M-th orders, respectively.
Although there are various methods using an orthogonal transform a discrete cosine transform (hereafter referred o as the DCT) s generally employed n the light of the coding efficiency. The transformation of the two-dimensional DC is given by an expression (2), end an inverse transform thereof is given by an expression (3). ##EQU1##
In addition, X(j, k) represents the elements of the pixel block 102, and j and k represent the positions of the elements. Y(u, v) represents the elements of the transformed coefficient 105, and u and v represent the positions of the elements.
The information reduction processing is performed by the coefficient selector 105 and the quantizer 107. In the coefficient selector 105, a coefficient is selected on the basis of a dispersion of the transformed coefficients 104 so as to obtain the selected transformed coefficient 106.
In such a transform coding method, a technique in which the dispersion of the coefficients is compared with a fixed threshold value to select a coefficient greater than the threshold and the coefficients lower than the threshold value are set to 0 to increase the compression efficiency is proposed in William K. Pratt: "Digital Image Processing", Wiley-Interscience, pp. 678-699. The threshold in this case can be determined from the statistics of the transformed coefficients of a multiplicity of images. In addition, a technique has been proposed in which the threshold is determined from the statistics of transformed coefficients for each image. However, in the case where the distribution of the transformed coefficients differs from these statistics, deterioration in image quality may occur.
In the quantizer 107, the selected transformed coefficients 106 are quantized to obtain the quantization coefficient 108.
As the code assignment processing, in the encoder 109, a code word is assigned to the quantization coefficient 108 so as to form the encoded data 110, which is outputted to the transmission line 111.
Through each processing described above, the image information can be coded by the transform coding method.
With the above-described method, however, since the selection of the coefficients is determined uniformly with respect to all the pixel blocks, there has been the problem that this method cannot provide adaptation to changes of a local nature of images.
To solve this problem, a technique for providing adaptation to each block has been proposed as disclosed in W. H. Chen et al.: "Adaptive Coding of Monochrome and Color Images," IEEE Transactions on Communications, Vol. COM-25, No. 11, pp. 1285-1292 (November 1977). In this technique, the image blocks are classified beforehand into four classes in accordance with the magnitude of an AC power in the block, and the standard for selecting a coefficient (bit assignment) is determined on the basis of the dispersion of the coefficients obtained for each class.
According to this method, since the classification is effected on the basis of only the relative magnitude of the AC power within the block, even if the AC power in the blocks is equal to each other, there occur differences in the distribution of coefficients owing to the directivity of edges and the like. With this method, however, since the coefficients at the same positions are simply selected in the same class irrespective of the differences in the distribution of the coefficients, it is impossible to select the most suitable coefficients for representing an input image, so that it has been impossible to obtain an image with sufficiently high quality.
In view of the above-described problems, a technique of classifying blocks using a vector quantizing technique has been proposed in Katoh, Takegawa, and Ohkubo: "Adaptive Orthogonal Transform Coding Method Using Classification," Transactions of the Institute of Electronics, Information and Communication Engineers (B), Vol. J71-B, No. 1, pp. 1-9, January 1988. It is reported therein that according to this method the image quality and performance are improved over the method disclosed in the above-described literature of W. H. Chen et al. by effecting a classification which takes into consideration the magnitude and the bias of the AC power in the blocks.
In the above-described adaptive orthogonal transform coding method using a classification, in order to reduce a huge amount of calculation entailed in the vector quantization, the coefficients used as vectors are restricted to low-frequency regions where the electric power is concentrated. For this reason, it is not considered that the coefficients are distributed up to a high-frequency region as in the case of pixel blocks including sharp edges. In addition, even if an attempt is made to effect a vector quantization including the coefficients in the high region by expanding the number of dimensions of the vectors, the high-frequency region coefficients rarely conform to a specific distribution. Hence, it is impossible to expect an effect of the classification.